In the chapter 957 titled “Ultimate” we got this panel of Gol D. Roger and Monkey D. Garp vs. Rocks D. Xebec:
So I realized that Roger’s sword looks almost identical to the sword Gryphon held by Shanks.
Gryphon is a durable and powerful sword, being capable of clashing equally with Whitebeard’s naginata, the Murakumogiri, which made the sky split apart.
When imbued with Haki, it was strong enough to block Sakazuki’s magma fist without taking any damage.
Unless Shanks has a near identical sword then I figure they’re the same. So Gryphon originally belonged to Gol D. Roger just like the straw hat.
*Theory by XraynPR